Add new cycling option
When using Xinerama, make it possible only to cycle between those
windows that are on the currently active head - AFAICT this means
windows that have the majority of their area on the same head as
the pointer.
Patch complete with WPrefs integration, alas this being quite
unusable - for a lack of a better idea, I tucked the option on the
expert panel, making the item list too tall - making it scrollable
would be a nice addition.
The default for the new option (CycleActiveHeadOnly) should be NO,
that is, no change in default behaviour.